This invention relates to improvements in clamp assemblies for steering column assemblies, especially but not exclusively to electric power assisted steering assemblies. In particular it relates to clamp assemblies for steering column assemblies adjusted for both rake and reach.
It is known to provide a steering column assembly in which a steering wheel is connected to one or more road wheels through a collapsible or telescopic steering column shaft. The shaft is designed to collapse away from a driver of the vehicle if a load is applied to the steering wheel in the event of an accident. The shaft may comprise a single part shaft, which has a weakened central section that will concertina under an axial load. Alternatively it may comprise a two part telescopic shaft in which one part slides axially into the other under an axial load.
To locate the steering shaft relative to the vehicle it is typically supported within one or more bearings carried by an adjustable support portion, often referred to as a steering column shroud. The shroud is in turn secured to a fixed portion bolted or otherwise fixed to a conveniently accessible part of the vehicle chassis or bulkhead.
In many vehicles produced today the column shroud, which may for convenience be thought of as a non-adjustable portion and an adjustable portion, is releasably locked to the fixed portion of the vehicle in a way that permits the steering shaft to be adjusted for rake (up and down) and reach (in and out). In some applications it may be adjustable only for rake, or only for reach. Where it is adjustable for reach and rake the shroud typically comprises an inner and outer tube which telescope relative to each other, one of the tubes being fixed and the other moving to allow for the required adjustment.
A lever—which must be located within easy reach of the driver—allows a clamping mechanism to be locked and unlocked for the position of the steering column shroud to be adjusted. This enables the driver of the vehicle to set the steering wheel at the most comfortable position.
With the introduction of steering wheel mounted airbags there has recently been a demand that, once adjusted, the column must be held in place by a positive locking method. By this we mean that, when the clamp mechanism is moved into its locked position, there must be greater restraint against subsequent column movement for reach than would be created solely by friction.
It is normal to increase the resistance to movement when clamped by providing for a toothed engagement between a rack of teeth on the fixed portion of the vehicle and a toothed block of the clamp assembly. The engagement between the teeth provides a positive locking of the shroud position. When unclamped the two racks of teeth are allowed to move apart so they can move relative to each other to permit rake adjustment.
Similarly it is known to provide a toothed engagement between a further toothed block on the clamp assembly and a corresponding rack of teeth on the adjustable portion of the shroud. When the clamp assembly is in the unclamped condition the two racks of teeth are allowed to move apart so they can move relative to one another. This permits adjustment for reach.
A basic problem with the use of teeth in this way is that on occasions the tips of the teeth on one rack may be exactly in line with the tips of the teeth of the other rack that should engage them when the driver attempts to move the clamp to the clamped condition. This so called tooth-on-tooth problem can either prevent the driver locking the clamp properly or in extreme cases damage the teeth or the locking mechanism. To resolve this it is known to allow one or more of the toothed blocks to be able to slide otherwise move by a small amount, perhaps up to one half of the tooth pitch of the racks, during clamping to permit the teeth of the racks to align correctly before reaching the clamped condition.
To move the teeth apart when moving to the unclamped condition it is known to provide the teeth of the clamp assembly on toothed blocks which are threaded onto a clamp pin of the clamp assembly, and to use a biasing means such as a coil spring that acts via a spacer freely mounted on the pin directly upon the toothed block. When clamped this biasing force is overcome by a clamp mechanism to urge the toothed block onto the corresponding toothed rack. When unclamped the biasing means urges the toothed block and the clamp pin away from the racks of teeth.
The applicant has found that the use of a high force of the biasing means acting on the toothed block can impair the free side to side movement of the block when clamped, leading to increased locking forces in instances where teeth are coincident.